Ignition distributor cap



' 1965 P. J. WILLSON ETAL 3,217,113

IGNITION DISTRIBUTOR GAP Filed Nov. 29, 1962 mm M2 Hm United States Patent 3,217,113 IGNITION DISTRIBUTOR CAP Philip J. Willson, Royal Oak, James E. Cserr, Warren,

and John C. Graham, Detroit, Mich., assignors to Chrysler Corporation, Highland Park, Mich., a corporation of Delaware Filed Nov. 29, 1962, Ser. No. 240,953 3 Claims. (Cl. ZOO-19) This invention relates to internal combustion engine ignition distributor caps and particularly to a coating applied to the cap interior to materially extend the life of the cap and to prevent failure of the cap during high humidity engine operation.

There has. been a long existent and bothersome problem of ditficult engine starting during rainy spells or under high humidity operating conditions which problem has been found to center in the ignition distributor. This problem has been aggravated in certain vehicle installations due to the fact that the distributor location in the vehicle engine compartment may have very little road water splash protection and/or air inlet apertures or ducting for the engine compartment tend to pass large volumes of moisture saturated air around the distributor unit such that moisture is depositied on the exterior of and introduced to the interior of the distributor cap. As a result of extensive tests run on distributor caps that were associated with hard engine starting problems, it has been determined that distributor cap conductivity resulting from deterioration of the interior surface of the phenolic cap material was a major cause of engine hard starting. This cap surface deterioration results in the cap interior surface becoming easily wetted by moisture in the surrounding air. When the interior surface of the distributor cap becomes wetted a conductive film may be established that shorts out adjacent spark plug terminals of the distributor cap and prevents the cap from functioning in its intended manner. Not only does the moisture film on the cap interior surface reduce the resistance of the cap, but in addition the moisture laden air within the cap fosters the formation of acids that attack the cap spark plug terminals and produce conductive salts that are deposited on the cap interior. The aforedescribed deteriorating action on the cap interior occurs in the following manner. The high voltage sparking which normally occurs within the distributor cap results in the formation of nitrogen oxides due to break down of the air within the cap. These oxides in the presence of any water inside the cap form nitric acid. Nitric acid and ozone, which is formed by the sparking occurring within the cap, cause deterioration of the phenolic cap material. Nitric acid attacks the aluminum spark plug terminal inserts which result in the formation of conductive salts within the cap. I In almost all cases of cap failure, a white deposit could be observed in varying amounts on the inside of the cap. This deposit when analyzed spectrochemically and by X-ray diffraction proved to be beta-Al O -H O, beta- Al O -3H O and Al(NO -H O. This was not unexpected and was easily explainable. Nitric acid is formed by electrical discharge through moist air, according to the following chemical reaction.

2NO+O 2NO 3NO +H O 2HNO +NO The nitric acid thus formed is able to attack the aluminum spark plug terminal inserts of the distributor cap, and nitric acid being an oxidizing agent, may oxidize the aluminum as well as the other alloy metals to one of its oxide forms, or may directly combine with the aluminum to form the nitrate.

3,217,113 Patented Nov. 9, 1965 As indicated from the foregoing description, distributor cap failure is due to two specific factors namely (1) chemical decomposition of the organic cap material and (2) formation of highly conductive aluminum salts inside the cap. After running numerous distributor caps to failure, it was evident that cap failure occurred in stages and these stages could be directly related to the wettability of the cap. If steam is allowed to condense in a new cap, tiny water beads form on the cap surface but, as the resistance of the cap decreases, the water droplets enlarge and finally the cap surface is wettable. The water is then able to form a continuous layer over the interior surface of the cap and, water being conductive, will conduct electricity across the surface of the cap and between the terminals thereof. Subsequently, it is only a matter of time before the cap surface is tracked by carbon paths and the cap fails completely. In a like manner, any salt formations that are formed within the distributor cap, such as aluminum nitrate, provide conductive paths and accelerate the time to cap failure after the cap has become wettab'le.

To prevent the occurrence of wettability of the cap interior surface various materials having hydrophobic properties were applied and tested as coatings for the interior surface of distributor caps. Several of the materials tested that exhibited good hydrophobic properties were epoxy, silicone resins, silicone rubbers, styrene, amorphous polypropylene, polyethylene, paraffin, polycarbonate and certain phenolic and polyacetal resins. In the cases where certain silicone rubbers were used as hydrophobic coatings it was found that if the cap interior surface was primed before application of the silicone rubber coating that then the life of the cap was significantly increased.

In preparing a distributor cap interior surface for coating with a hydrophobic material it has been found beneficial to first vapor blast the cap interior with a fine sand and water spray for approximately two minutes. The cap is then rinsed with tap water and dried in a stream of pressurized air. After drying the cap is primed with a primer compatible with the coating to be applied and thereafter the interior of the cap is coated, preferably by a spray method, with a fil'm of material exhibiting hydrophobic properties. Amorphous polypropylene has proven to be an excellent coating and it does not require a primer coating.

Now looking at the drawings:

FIG. 1 is a top plan view of a distributor cap for an eight cylinder spark ignition internal combustion engine;

FIG. 2 is a sectional elevational view taken along the line of and in the direction of the arrows 22 of FIG. 1;

FIG. 3 is a view similar to FIG. 2 showing the distributor cap in an inverted position and in the process of having the hydrophobic coating applied to the inside surface thereof.

The distributor cap C embodying this invention comprises a cup-shaped element 20 formed of an insulating plastic material such as a phenol-formaldehyde resin. The closed bottom end 21 of the cup 20 has a centrally positioned tower 23 projecting outwardly therefrom. This central tower 23 has seated therein and projecting through the cup bottom 21, a tubular contact terminal 24. Terminal 24 is adapted to receive one end of a wire conductor element that is adapted to be connected to the high tension winding of an ignition coil (not shown). Extending about the periphery of the cup bottom 21 and projecting outwardly therefrom are eight circumferentially spaced towers 26 that each seat a tubular contact terminal 27. The lower ends 28 of the terminals 27 project through the bottom 21 of the cup 20 to provide the stepped terminal portions for intermittent spark con nection with the contact 30 of the distributor rotor 31. Rotor 31 is carried by rotor shaft 32 that is drivingly connected to the associated engine camshaft (not shown). The terminals 27 are each adapted to receive a Wire conductor that is connected to one of the spark plugs of the associated engine.

Looking at FIGS. 2 and 3, it will be noted that a plastic coating 40 has been spray applied to the entire interior surface of the cap C. A conventional spray head 41 can be used to apply a relatively thin layer of a hydrophobic coating material to the interior of the cap C. From FIGS. 2 .and 3 it will be seen that the plastic coating 40 not onlycovers the interior surface of the plastic cap material 20, but in addition, the coating 40 covers the exposed, stepped ends 28 of the contact terminals 27. With certain coating materials like silicone rubbers, it is necessary to prepare the cap surface for the coating and to also remove the coating material from the stepped contact surfaces 28 of the terminals 27 and from the contact surface 42 of the high tension terminal 24 before the cap C is usable for its intended purpose. However, With amorphous polypropylene as a coating material, no surface preparation is required before application of the coating and furthermore it is not necessary to remove this coating material from the terminal contact surfaces before placing the cap in service. Another advantage for amorphous polypropylene as a cap interior surface coating appears to be that it is less costly than most of the other coating materials that have been tested. It has been found from tests conducted on distributor caps coated in accordance with this invention that a coating thickness of between .005 inch and .0015 inch was satisfactory to prevent breakdown of the cap material and to thereby retain the electrical resistance of the cap and its resistance to wettability on the interior surface of the cap. The exact minimum thickness that can be used is that which will insure a continuous coated surface and at the same time give good adherence to the interior surface of the cap C. One of the interior surface coatings that has proven to be most satisfactory and one that provides an electrically insulative, hydrophobic, chemical material that is easily applied without cap surface preparation and which adheres to the cap interior surface under all normal operating conditions, is a solution of amorphous polypropylene consisting of fifty (50) parts by weight of xylene or 1,1,1 trichlorethane solvent with to parts by weight of amorphous polypropylene.

Life of coated caps appears to be at least four times better than uncoated caps and in addition the life of the distributor contact points (not shown) that are enclosed by the cap. C also appears to be materially improved when the short circuit sparking that can occur within uncoated caps is eliminated.

While FIG. 3 of the drawings shows the hydrophobic coating material 40 being applied by a spray method, still, it is to be understood that any method of coating application can be used that is compatible with the particular coating material used. Painting, swabbing, dipping or any other method of surface coating that will apply a relatively thin, continuous, surface coating to the cap C interior surface is within the contemplation of this invention. Furthermore, while the primary beneficial effect results from application of the coating to the interior surface of the cap C, still, it may also be beneficial to coat both the interior and exterior surfaces of the cap C With a chemically inert, electrically insulating, hydrophobic material that is easy to apply as a thin continuous coat ing to material that might be used for an ignition distributor cap.

While the silicone rubber coatings seem to necessitate removal of the coating from the contact terminals 28 before utilization of the coated distributor cap, the coating of amorphous polypropylene does not require its removal from the terminal contacts 28 because even though the coating is an electrical insulator, still the ignition spark from the rotor 30 to the contacts .28 is able to penetrate the coating and to fire the associated spark plugs. Three (3) additional factors that make amorphous polypropylene especially well suited for application as a spank ignition distributor cap coating are:

(1) Adhesive character.-Contrary to other (higher molecular weight) forms of polypropylene, amorphous polypropylene adheres well to most surfaces.

(2) Low melting point.-Although a fair degree of temperature resistance is necessary for this application, the relatively low melting point of amorphous polypropylene provides certain properties which make it particularly well suited for this application. The heat from the electrical discharge will melt the coating off of the spark plug inserts so that it does not interfere in any Way with the normal operation of the engine. The relatively low melting point also permits healing of minor damage at operating temperatures.

(3) Elastic character.'BeCause of the low molecular Weight of amorphous polypropylene, it does not become brittle, even at low temperatures; therefore, there is little danger of it chipping off from the cap surfaces.

We claim:

1. A spark ignition distributor cap formed from an insulating resin material that mounts spaced apart conductor terminals on the inner surface of said cap, said inner surface of said cap being provided with a coating of a chemically inert, hydrophobic, electrically insulative material that will resist wettability of the cap inner surface, said coating being one that resists the deterioration of the cap inner surface due to electrical stress within the cap producing ozone and oxygen and said coating consisting of a hydrocarbon soluble .polyolefin selected from a group consisting of polypropylene and polyethylene suspended in a solvent selected from a group consisting of a chlorinated hydrocarbon and an aromatic hydrocarbon with said coating being of such a consistency to be applicable as a continuous cap surface coating of between approximately .0015 and -005 inch thickness.

2. A spark ignition distributor cap formed from an insulating resin material that mounts spaced apart conductor terminals on the inner surface of said cap, said inner surface of said cap being provided with a coating of a chemically inert, hydrophobic, electrically insulative material that will resist wettability of the cap inner surface, said coating being one that resists the deterioration of the cap inner surface due to electrical stress within the cap producing ozone and oxygene and said coating consisting of amorphous polypropylene prepared in a solution of 5 to 15 parts by weight of amorphous polypropylene to 50 parts by weight of xylene.

3. A spark ignition distributor cap formed from an insulating resin material that mounts spaced apart conductor terminals on the inner surface of said cap, said inner surface of said cap being provided with a coating of a chemically inert, hydrophobic, electrically insulative material that will resist wettability of the cap inner surface, said coating being one that resists the deterioration of the cap inner surface due to electrical stress Within the cap producing ozone and oxygen and said coating consisting of amorphous polypropylene prepared in a solution of 5 to 15 parts by weight of amorphous polypropyllene to 50 parts by weight of trichloroethane.

References Cited by the Examiner UNITED STATES PATENTS 2,064,052 12/ 36 Arthur l74211 X 2,365,925 12/44 Zoerlein et al 20019' 3,077,514 2/63 Kang 174-110 X 3,091,536 5/63 Rusignuolo et al.

3,096,001 7/63 Boe et al. 26033.6 3,145,111 8/64 Norton.

BERNARD A. GILHEANY, Primary Examiner. 

1. A SPARK IGNITION DISTRIBUTOR CAP FORMED FROM AN INSULATING RESIN MATERIAL THAT MOUNTS SPACED APART CONDUCTOR TERMINALS ON THE INNER SURFACE OF SAID CAP, SAID INNER SURFACE OF SAID CAP BEING PROVIDED WITH A COATING OF A CHEMICALLY INERT, HYDROPHOBIC, ELECTRICALLY INSULATIVE MATERIAL THAT WILL RESIST WETTABILITY OF THE CAP INNER SURFACE, SAID COATING BEING ONE THAT RESISTS THE DETERIORATION OF THE CAP INNER SURFACE DUE TO ELECTRICAL STRESS WITHIN THE CAP PRODUCING OZONE AND OXYGEN AND SAID COATING CONSISTING OF A HYDROCARBON SOLUBLE POLYOLEFIN SELECTED FROM A GROUP CONSISTING OF POLYPROPYLENE AND PROLYETHYLENE SUSPENDED IN A SOLVENT SELECTED FROM A GROUP CONSISTING OF A CHLORINATED HYDROCARBON AND AN AROMATIC HYDROCARBON WITH SAID COATING BEING OF SUCH A CONSISTENCY TO BE APPLICABLE AS A CONTINUOUS CAP SURFACE COATING OF BETWEEN APPROXIMATELY .0015 AND .005 INCH THICKNESS. 